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Conservation tillage effectively protects the environment. A no-till planter can seed on the straw mulching farmlands, and it is the most important machine in conservation tillage. In order to make sure the consistency of sowing depth, an active seeding depth control system was designed for a no-till planter. The system was consisted of piezoelectric film sensor, a signal processing circuit, a control signal generating circuit and a pneumatic drive system. When planting with a conventional row crop planter which had a plurality of row units mounted on a toolbar, it was necessary to provide a sufficient down force on each row unit to ensure that the seed trench opener was fully penetrated into the soil. Proper down force was also essential to provide proper amount of soil compaction in the side walls of the seed trench. Seeding depth primarily depended on the down force on each row unit. In order to make adjustments to the down force in real time, it was needed to measure the magnitude of the down force on an ongoing basis. The piezoelectric film sensors were used for measuring the deformation of depth limit wheel, the down force of the row unit was monitored in real time according to the deformation of the limit wheel. The piezoelectric film generated charge during rotating of the wheel. The sensor changed the charge into a voltage signal. The voltage signal was processed by low-pass filter and preamplifier circuit in order to eliminate the noise and amplifiy the filter, and then the peak-holding circuit gained the peak voltage value which was sampled by A/D conversion and sent to control signal generating circuit by the wireless transmitter. Wireless receiver received the digital signal which was compared with the present value stored in the microcontroller, if the received signal was less than the present value, the difference between the received signal and the present value formed the control signal. The pneumatic drive system was consisted of air compressors, tank, filters, electric-gas proportional valve and air spring. To provide the necessary down force, an air spring was assembled at the linkage which was mounted in the row unit to the toolbar. Compressed air was generated by the air compressor and stored in the tank, and then the compressed air was filtered to filter out dust and impurities. Pure compressed air was fed to the electric-gas proportional valve; the output of electricgas proportional valve was connected to the air spring. The output gas pressure of the electricgas proportional valve was controlled by control signal, the air spring can generate corresponding down force on the unit row. A field experiment was conducted to verify the performance of the seeding depth automatic control system, and the result showed that the seeding depth control system can provide accurate ditching depth and reliable performance of profiling. When the speed of the planter was over 8km/h, the qualified sowing depth rate of the active seeding depth control system was much better than that of the passive control system.